The present invention relates to a vehicular steering angle estimating apparatus for estimating a steering angle based on a rotational speed of a wheel (wheel speed), a self aligning torque (SAT) and a rotational angle of a motor and an electric power steering apparatus mounted therewith, particularly relates to a vehicular steering angle estimating apparatus capable of detecting a change in a steering characteristic of a vehicle and capable of accurately estimating an actual steering angle by preventing an erroneous estimation of the steering angle by detecting a state in which the wheel is not suitable for the estimation of the steering angle and an electric power steering apparatus mounted therewith.
An electric power steering apparatus for urging to assist a load of a steering apparatus of a vehicle by a rotational force of a motor urges to assist of a load of a steering shaft or a rack shaft by a driving force of the motor by a transmission mechanism of a gear or a belt or the like by way of a speed reducing machine. Such an electric power steering apparatus of a background art carries out a feedback control of a motor current in order to accurately generate an assist torque (steering assist force). The feedback control adjusts a voltage applied on the motor such that a difference between a current instruction value and a detected value of the motor current is reduced and the voltage applied on the motor is adjusted generally by adjusting a duty ratio of a PWM (pulse width modulation) control.
Here, explaining by showing a general constitution of an electric power steering apparatus in FIG. 15, a column shaft 2 of a steering wheel 1 is connected to a tie rod 6 of a steered wheel by way of a speed reducing gear 3, universal joints 4a and 4b and a pinion rack mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 is connected to the column shaft 2 by way of the speed reducing gear 3. A control unit 30 for controlling the power steering apparatus is supplied with a power from a battery 14 and inputted with an ignition signal by way of an ignition key 11, the control unit 30 calculates a steering assist instruction value I of an assist instruction based on a steering torque Th detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12 and controls a current supplied to the motor 20 based on the calculated steering assist instruction value I.
A steering mechanism is constructed by a constitution as shown by FIG. 16, the motor 20 exerts a rotational force (assist torque) to the column shaft 2 by way of a speed reducing gear (worm gear) 3, and an output of a motor rotational angle sensor 21 attached to the motor 20 and output of the torque sensor 10 attached to the column shaft 2 are inputted to the control unit 30. Further, the control unit 30 controls to drive the motor 20.
The control unit 30 is mainly constituted by CPU (including MPU or MCU), and FIG. 17 shows a general function executed by a program at inside of the CPU.
Explaining a function and an operation of the control unit 30 in reference to FIG. 17, the steering torque Th detected by the torque sensor is inputted to a steering assist instruction value calculating portion 32, and also the vehicle speed V detected by the vehicle speed sensor 12 is inputted to the steering assist instruction value calculating portion 32. The steering assist instruction value calculating portion 32 determines the steering assist instruction value I constituting a control target value of the current supplied to the motor 20 in reference to an assist map stored to a memory 33. The steering assist instruction value I is inputted to a subtracting portion 30A and inputted to a differential compensating portion 34 of a feed forward system for increasing a response speed, a deviation (I−i) of the subtracting portion 30A is inputted to a proportional operating portion 35 and inputted to an integral operating portion 36 for improving a characteristic of the feedback system, and a proportional output thereof is inputted to an adding portion 30B. Also outputs of the differential compensating portion 34 and the integral compensating portion 36 are inputted to add to the adding portion 30B, and a current control value E constituting a result of addition at the adding portion 30B is inputted to a motor driving circuit 37 as a motor drive signal. The motor driving circuit 37 is supplied with a power from the battery 14, a motor current value i of the motor 20 is detected by a current detecting portion 38 and the motor current value i is inputted to be fed back to the subtracting portion 30.
According to the electric power steering apparatus, it is necessary to detect or estimate a steering angle (steering angle) from a necessity of grasping a state of steering, and although the steering angle can be detected accurately when the steering angle is detected by providing a steering angle sensor, cost is increased by an amount of the steering angle sensor. Therefore, various methods of estimating a steering angle from a wheel rotational speed have been proposed in a background art. Actually, when the steering angle is estimated from only a geometrical relationship of a vehicle, there is a concern that an estimation error is brought about or the steering angle is erroneously estimated by steering, a road surface state or the like.
For example, JP-A-2005-98827 (Patent Reference 1) proposed a vehicular steering angle estimating apparatus for resolving such a problem. The apparatus disclosed in Patent Reference 1 is a vehicular steering angle estimating apparatus provided with wheel rotational angle sensors respectively for four wheels for estimating a steering angle of a vehicle based on the wheel rotational speeds from the respective wheel rotational speed sensors for detecting slip of the four wheels by comparing a relationship between the wheel rotational speeds of respective two wheels of front and rear and left and right wheels.    [Patent Reference 1] JP-A-2005-98827
However, according to the vehicular steering angle estimating apparatus disclosed in Patent Reference 1, the rotational speeds of four wheels are needed to pose a problem that the apparatus cannot deal with the vehicles providing only rotational speeds of two wheels (front wheels or rear wheels).